In order to exchange data in a unidirectional or bidirectional manner, the smart cards require a regulated voltage supply Vout capable of delivering a certain current Iout, from an input voltage Vup. This supply, for which an embodiment is described in the FIG. 1, is generally delivered by an interface circuit contained in a smart-card reader.
The voltage supply which is delivered to the smart card is a stabilized supply whose output level is regulated to a certain value compatible with the characteristics of the smart card. In general, this regulation of the voltage allows to guarantee an output voltage equal to a target voltage Vcons, with a margin or error of a few percent.
In parallel, the supply to the interface circuit comprises control means to generate a change of state of a second control signal SC2 when the current flowing in the smart card exceeds a certain threshold value, for example to forewarn a possible short-circuiting in the smart card.
However, this interface circuit has functional limitations.
In normal operating conditions, the input voltage Vup must remain greater than the target voltage Vcons so that the regulation of the voltage is done correctly, and therefore a correct supply to the smart cards is guaranteed. If for any reason, the input voltage Vup just drops below the target voltage Vcons, then the output voltage Vout also drops without any control signal being generated. As this drop in voltage is not detected, it can be detrimental to the operation of the smart card or for the application using the smart card.
On the other hand, in case of a large drop of the input voltage Vup, the output current Iout which is flowing in the smart card is no longer significant as the smart card is no longer supplied by the correct output voltage Vout. The control means which generate the second control signal SC2 can then no longer play their role. If a short-circuit in the smart card occurs at this moment, the short-circuit has the risk of not being detected and runs the risk of deteriorating the application using the smart card.